1. Field of the Invention
The invention relates to an illumination system for a microlithography projection exposure apparatus for illuminating an illumination field with the light from an assigned light source, and to a method for illuminating an illumination field, in particular with such an illumination system.
2. Description of the Related Art
The performance of projection exposure apparatuses for the microlithographic fabrication of semiconductor components and other finely patterned devices is substantially determined by the imaging properties of the projection objectives. Furthermore, the image quality and the wafer throughput that can be achieved with the apparatus are substantially co-determined by properties of the illumination system arranged upstream of the projection objective. Said illumination system must be able to prepare the light from a primary light source, for example a laser, with the highest possible efficiency and in the process to generate an intensity distribution that is as uniform as possible in an illumination field of the illumination system.
A high degree of uniformity or homogeneity can be achieved by mixing the light coming from the light source with the aid of a light mixing device. In the case of light mixing devices, a distinction is made essentially between light mixing devices with fly's eye condensers and light mixing devices with integrator rods or light mixing rods.
Moreover, it is intended to be possible to set different illumination modes at the illumination system in order, for example, to optimize the illumination in accordance with the structures of the individual originals to be imaged (masks, reticles). Pupil shaping units are provided for setting the illumination modes or illumination settings, which pupil shaping units can generate a predeterminable light distribution in a pupil plane.
In the case of illumination systems which, for setting different illumination modes, operate with exchangeable optical elements (e.g. diffractive optical elements or spatial filters), the number of different illumination settings is limited by the number of different elements that can be exchanged. If the intention is that a large number of illumination settings can be set at such systems, a multiplicity of exchangeable optical elements must be made available, the production of which is complicated and may be associated with considerable costs.
The use of digital filters in illumination systems is known for various purposes. The filters may be positioned e.g. as variable aperture diaphragms or as pupil shaping elements for setting a predeterminable light distribution in a pupil plane. A pupil shaping exclusively by means of filters is always associated with loss of light, however, particularly if relatively large regions of the light distribution incident on the filter have to be masked out for producing off-axis settings.
U.S. Pat. No. 6,215,578 describes a wafer stepper comprising an illumination system for generating abaxial illumination settings. The illumination system has an electronically drivable, first digital filter for setting illumination settings. Arranged downstream of said first digital filter is a second digital filter, which is positioned on the illumination field of the illumination system and generates there a predeterminable mask pattern that is imaged onto a wafer through a projection objective arranged downstream. Both digital filters can be driven jointly by means of a computer, thereby enabling the light distribution generated by the pupil shaping element to be adapted to the mask structure.
U.S. Pat. No. 5,707,501 shows a projection exposure apparatus which, in one embodiment, comprises a transmission filter designed as a liquid crystal array. Said transmission filter is arranged before a fly's eye condenser in the illumination system of the projection exposure apparatus and serves as a pupil shaping element in order to set e.g. annular illumination.
JP 06216007 describes a projection exposure apparatus having an illumination system in which a liquid crystal digital filter is arranged in the region of a pupil plane in the light path behind a fly's eye condenser. The digital filter is electrically drivable and serves as an adjustable diaphragm for limiting the beam bundle diameter.
JP 10022209 describes a filter which comprises an array of liquid crystals and is designed for use in an illumination system of a projection exposure apparatus. The filter serves as a transmission filter with location-dependent transmission that can be set variably and permits the setting of desired two-dimensional intensity distributions. The resolution and the depth of focus of the projection are thus intended to be able to be influenced.
U.S. Pat. No. 6,281,967 shows an illumination system for a microlithographic projection exposure apparatus having a rod integrator. In the light path before the rod integrator there is provided a transmission filter element for setting the intensity distribution of the light falling into the rod integrator. The filter element can be exchanged as required for filter elements having a different transmission distribution. A turret plate is arranged in a pupil plane behind the rod integrator, there being fitted in said turret plate a plurality of aperture arrangements which in each case have openings of different size or form and serve for pupil shaping.
U.S. Pat. No. 6,051,842 describes an illumination system having a fly's eye condenser, upstream of which a variable optical filter is arranged. The filter serves as a variable attenuator for homogenizing the light distribution in a field plane upstream of the fly's eye condenser.
The documents JP 12150375, JP 11312639 and JP 11054417 show embodiments of filters which all serve for homogenizing the light distribution in a field plane. In this case, the location-dependent filter effect is fixedly predetermined and can only be altered by a filter change.